Various linear, thermotropic polymers are known. These also include polyesters or polyester amides, which form an anisotropic melt and, through processing from this melt, lead to products with particularly advantageous mechanical properties, such as high strengths and a high modulus (see for example European patents Nos. 63,880, 67,032, 81,900, and 7,715, and U.S. Pat. Nos. 4,393,191, 4,184,996, 4,473,682, 4,351,917, and 4,272,625). The molded objects, films and yarns of thermotropic polymers are, of course, stable only up to temperature below their polymer melting point. Therefore, attempts were made to fix the shape of liquid crystalline polyesters after they were processed, while retaining their advantageous properties, either by drastically increasing the melting point or by making the polymers infusible. A known method involves the subsequent tempering of the processed polymer at temperatures just below the melting point for several hours. This leads to an increase in molecular weight and thus also to higher melting points (see for example U.S. Pat. No. 4,066,620, 4,473,682, 4,499,259 and 4,500,699).
A different possibility is the fixation of the polymers by crosslinking. Fully aromatic, thermotropic polyesters which, after being molded, are cross linked oxidatively by being heated in an oxygen atmosphere, are disclosed e.g. in U.S. Pat. No. 4,224,433. The degradation reactions of the polymer, which take place at the same time, are a disadvantage. In U.S. Pat. No. 4,350,809, polymers including polyesters are described, which form anisotropic melts and have so-called "self cross linking" reactants in the polymer chain. These are monomeric components generally with three or more reactive hydroxyl, carboxyl, amino, isocyanate or carodimide groups or combinations of these. In U.S. Pat. No. 4,147,043, polymers are disclosed, which are synthesized by addition polymerization of monoethylenically unsaturated monomers, and contain compounds with two ethylenic, unsaturated groups, such as p-divinylbenzene, allyl acrylate, diallyl maleate or fumarate, etc., as cross linking agents. Only a very small amount of cross linking agent can be used (0.01 to 5%), since the polymers cross link already during the synthesis and higher amounts lead to a solid, which cannot be processed further.
Furthermore, liquid crystalline, fully aromatic polymers with stilbene or tolan units in the main polymer chain are disclosed e.g. in U.S. Pat. No. 4,654,412, which can be cross linked by electron acceptor monomers, such as maleic acid, maleic anhydride, fumaric acid, or itaconic acid. The high melting and processing temperatures, as well as the high melt viscosity of fully aromatic, liquid crystalline polyesters are disadvantageous. Moreover, the long cross linking times (20 hours) at elevated temperatures and the subsequently required tempering of several hours are a disadvantage.
Liquid crystalline polyesters and polyester amides with fumaric acid units in the main polymer chain are disclosed in the U.S. Pat. No. 4,398,019. Further components are aliphatic dicarboxylic acids, aromatic hydrocarboxylic acids, diphenols and p-aminophenols. These polymers are random copolymers with various sequences. They are not suitable for cross linking reactions with retention of the liquid crystalline properties, since the stiffness is lost when the C.dbd.C double bond is split.
Methods to synthesize liquid crystalline polymers of regular structure with rigid and flexible units in the main polymer chain, which contain a reactive unsaturated group in the form of fumaric acid in the mesogenic group, are known from Makromol. Chemie, Rapid Communication vol. 6 (1985), No. 9 page 601. The disadvantage of this polyester is in that a reaction of the C.dbd.C double bond to a single bond, as would be the case with addition or cross linking reactions, leads to the loss of liquid crystalline properties due to a destabilization of the mesogenic group (Vysokomol. Soedin. B 24 (1987) 7, page 504). Likewise, liquid polyesters of regular structure, with long, saturated, uniform spacers, such as 1,10-dioxydecane or oligooxypropylene are known. They have a low melting point that is very advantageous for processing, but only have a limited range of applications because of their low melting point (Vysokomol. Soedin A 26 (1984) 12, page 2570).
No methods or means are disclosed in the known state of the art, as to how an unsaturated, liquid crystalline polyester amide of regular structure and having a sufficiently low melting point can be synthesized and cross linked with retention of the liquid crystalline properties.